The present disclosure relates to semiconductor devices and, more particularly, to semiconductor devices including a gate structure and methods of forming the same. With their miniaturization, multi-functions, and/or low manufacturing costs, semiconductor devices have been in the limelight in the electronics industry. Semiconductor devices may be divided into semiconductor memory devices for storing data, semiconductor logic devices for calculating logic data, and hybrid semiconductor devices including memory elements and logic elements. As the electronics industry has rapidly developed, requirements for characteristics of the semiconductor devices have increased. For example, requirements for high reliability, high speed, and/or multi-functions of the semiconductor devices have increased. To satisfy these requirements, structures of the semiconductor devices have become more complicated. Also, the semiconductor devices have continuously become more highly integrated.